There are two types of electron-emitting devices known heretofore, thermionic emission sources and cold-cathode emission sources, and there are also the known image-forming apparatus making use of these electron sources.
The image-forming apparatus illustrated in FIG. 11 is known as a plane type image-forming apparatus using the thermionic emission source. FIG. 11 is a schematic structural diagram of the image-forming apparatus using the conventional thermionic emission source.
This image-forming apparatus has a plurality of anodes 1502, which are arranged in parallel on an insulating substrate 1501 and the surface of which is coated with a material that emits fluorescence upon collision of an electron beam therewith (phosphor), a plurality of filaments 1503, which are arranged in parallel and opposite to the anodes 1502, and a plurality of grid electrodes 1504, which are arranged perpendicular to the anodes 1502 and filaments 1503 between the anodes 1502 and the filaments 1503, and these anodes 1502, filaments 1503, and grid electrodes 1504 are held in a transparent vessel 1505. The vessel 1505 is hermetically bonded (hereinafter referred to as “sealed”) to the insulating substrate 1501 so as to be able to keep the inside in vacuum, and the inside of the envelope constructed of the vessel 1505 and the insulating substrate 1501 is kept in the vacuum of about 1.3×10−4 Pa.
The filaments 1503 emit electrons when heated in vacuum and, with application of respectively appropriate voltages to the grid electrodes 1504 and to the anodes 1502, the electrons emitted from the filaments 1503 collide with the anodes 1502, whereupon the phosphor on the anodes 1502 emits fluorescence. Light-emitting positions can be controlled by matrix addressing of the lines of anodes 1502 (in the X-direction) and the lines of grid electrodes 1504 (in the Y-direction), whereby an image can be displayed through the vessel 1505.
The image-forming apparatus using the thermionic emission source, however, has the following problems: (1) power consumption is large, (2) it is difficult to implement large-capacity display because of slow modulation speed, and (3) variation occurs readily among the devices, and it is not easy to realize a large screen, because the structure becomes complex. Thus there are also the image-forming apparatus using the cold-cathode emission source instead of the thermionic emission source.
The cold-cathode emission sources include field emission type (hereinafter referred to as “FE type”), metal/insulator/metal type (hereinafter referred to as “MIM type”), surface conduction electron-emitting devices, and so on.
Examples of the known FE type devices are those described in W. P. Dyke & W. W. Dolan, “Field emission”, Advance in Electron Physics, 8, 89 (1956), or in C. A. Spindt, “Physical Properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5248, (1976), and so on.
An example of the image-forming apparatus using this FE type electron source will be described referring to FIG. 12. FIG. 12 is a schematic, structural diagram to show the conventional image-forming apparatus with the FE type electron source, partly enlarged.
As illustrated in FIG. 12, this image-forming apparatus has an electron source 2001, in which many electron-emitting devices are formed, and a face plate 2003 opposed to the electron source 2001. The electron source 2001 is comprised of a lot of micropoints 2013, which are formed in an electrically connected state through electric conductors 2012 on an insulating substrate 2011, and a grid 2015, which has apertures corresponding to the micropoints 2013 and which is supported on the insulating substrate 2011 while being electrically insulated from the micropoints 2013 by insulating layer 2014. The bottoms of the micropoints 2013 have the diameter and height of about 2 μm and the diameter of the apertures in the grid 2015 is also about 2 μm.
The face plate 2003 is comprised of the phosphor 2032, which is laid on the inner surface of glass sheet 2031, and an electroconductive film 2033, which covers the phosphor 2032 and which acts as an acceleration electrode to which a voltage for accelerating electrons emitted from the micropoints 2013 is applied.
In the above structure, the distance is very small between the tips of the micropoints 2013 and the grid 2015 (not more than 1 μm), and the tips of the micropoints 2013 are of a pointed shape. Therefore, a strong electric field (not less than 107 V/cm) capable of field electron emission can be created between the micropoints 2013 and the grid 2015 even by the potential difference of not more than 100 V. The amount of electron emission from one micropoint 2013 is approximately several μA. Since it is possible to form approximately several ten thousand micropoints 2013 per mm2, an electron-emitting device corresponding to one pixel is normally composed of a set of about several thousand to several ten thousand micropoints 2013 in the image-forming apparatus. Therefore, the electron emission amount can be over several mA per electron-emitting device corresponding to one pixel.
The potentials at the grid 2015 and at the micropoints 2013 are set, for example, as follows: the earth potential (0 V) is applied to the grid 2015 and a negative potential (about −100 V) is applied through the conductor 2012 to the micropoints 2013, which implements electron emission. Further, a potential equal to or greater than that at the grid 2015 is applied through the conductive film 2033 to the face plate 2003, whereby the electrons emitted from the electron source 2001 come to collide with the phosphor 2032 to excite the phosphor and effect light emission thereof.
For controlling luminous points of this emission, there are provided a plurality of row wires 2041 formed of an array of X-directional beltlike conductors 2012, each being electrically connected to a plurality of micropoints 2013, and column wires 2042 of the grid 2015 electrically connected in the Y-direction, and an image can be displayed in such a manner that matrix addressing is implemented so as to apply a voltage over a desired electron emission start voltage to desired areas out of a plurality of electron-emitting device areas 2010 formed at intersections of this matrix wire pattern from external power supplies 2043, 2044, thereby selecting positions where the electrons impinge upon the phosphor 2032 to which the voltage is applied through the conductive film 2033 from an acceleration voltage supply 2045.
On the other hand, examples of the known MIM devices are those described in C. A. Mead, “Operation of Tunnel-emission Devices”, J. Appl. Phys., 32,646 (1961) and so on.
Examples of the surface conduction electron-emitting devices are those described in M. I. Elinson, Radio Eng. Electron Phys., 10, (1965) and so on.
The surface conduction electron-emitting devices are the electron-emitting devices making use of the phenomenon that electron emission occurs when electric current flows in parallel to the surface in small-area thin film formed on a substrate. The surface conduction electron-emitting devices reported heretofore include those using thin films of SnO2 reported by aforementioned Elinson et al., those using thin films of Au [G. Dittmer: “Thin Solid Films,” 9,317 (1972)], those using thin films of In2O3/SnO2 [M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.”, 519 (1975)], those using thin films of carbon [Hisashi Araki et al.: Vacuum, vol 26, No. 1, p22 (1983)], and so on.
FIG. 13 is a plan view of the device reported by aforementioned M. Hartwell et al., which is a typical example of the device configuration of these surface conduction electron-emitting devices. In the same figure numeral 3001 designates a substrate and 3004 an electroconductive thin film made of a metallic oxide by sputtering. The conductive thin film 3004 is formed in a plane shape of H-pattern as illustrated. An electron-emitting region 3005 is made by an energization process called energization forming described hereinafter, in the conductive thin film 3004. In the figure the clearance L between the device electrodes is set to 0.5 to 1 mm and W to 0.1 mm. For convenience' sake of illustration, the electron-emitting region 3005 is illustrated in the rectangular shape in the center of the conductive, thin film 3004, but this is just a schematic illustration, which does not always loyally represent the position and shape of the actual electron-emitting region.
In the above-stated surface conduction electron-emitting devices including the device by M. Hartwell et al., it was common practice to form the electron-emitting region 3005 by subjecting the conductive thin film 3004 to the energization process called energization forming before execution of electron emission. Namely, the energization forming is a process of placing a constant, direct current or a direct current with increasing voltage at a very slow rate, for example, of about 1 V/min between the both ends of the conductive thin film 3004 to energize it, so as to locally break or deform or modify the conductive thin film 3004, thereby forming the electron-emitting region 3005 in an electrically high resistance state.
A fissure is created in part of the locally broken or deformed or modified, conductive thin film 3004. When an appropriate voltage is applied to the conductive thin film 3004 after the energization forming, electron emission occurs near the fissure.
Since the cold-cathode emission sources described above can be made by the technology, for example, such as photolithography, etching, and the like, it is feasible to place many devices at small intervals. In addition, the cathodes and surroundings can be driven under relatively lower temperature conditions than in the case of the thermionic emission sources, and thus multiple electron beam emission sources can be readily realized at finer array pitch. Among these cold-cathode emission sources, the surface conduction electron-emitting devices are particularly suitable for the electron-emitting devices used in the large-screen image-forming apparatus desired recently, because they are advantageous in that the device structure is simple and easy to produce and in that it is easy to produce a large-area screen.
For example, a known image-forming apparatus using the electron-emitting devices of this type is constructed in such structure that an electron source with the electron-emitting devices formed therein is opposed through a support frame to an image-forming member equipped with the phosphor or the like emitting fluorescence upon collision of electrons therewith and that the inside of an envelope composed of these electron source, image-forming member, and support frame is kept in vacuum.
The image-forming member is provided with the acceleration electrode for accelerating the electrons emitted from the electron source toward the image-forming member, and the emitted electrons are accelerated toward the image-forming member with application of high voltage to the acceleration electrode, to collide with the image-forming member. Therefore, the support frame is made of an insulating material resistant to the high voltage.